Transformer Selection

Good Day,

I want to buy a mains transformer and I noticed that some have output specified as

1A 0-15V AC

and another has
1A 15 - 0 - 15V AC

What does this second case mean? why the extra fifteen?

You can use the extra 15V to create another 15V output so you have two 15V outputs capable of delivering 1A. The other use is, that you can use it to make a powersupply with both +15V and -15V if you need negative voltage for your project.

Ok, you will of course have a drop in the rectifier, so you won't get +/-15V. A quick search got me this: 12 Vdc Dual Power Supply circuit with IC 7812&7912 – Free Electronic Circuit Schematics which illustrates what I mean.

Generally in your second case it would be a center tapped 30 volt secondary. Therefore you see 15 - 0 - 15. Attached is an image of what it looks like with a center tap. However, each 15 volts is 1/2 the rated current.

You can also use just two diodes to get full wave rectification like this:-
File:Fullwave.rectifier.en.png - Wikipedia, the free encyclopedia

I used to do that on a lot of power supplies, but you there are more losses in the transformer than if you use a full bridge.

Many transformers have two windings of the same rating so that you can put them in series or parallel so they can be either 0-15V or 15-0-15V, such as this Farnell Export